The analytical investigation of problems arising from the study of experimental results on circadian rhythms: (1) Populations of interacting oscillators as related to rhythm splitting, multiple states of synchrony, nonmonotonic transients and zonation patterns (in cultures of fungi). (2) The effects of light intensity on the free run period and their relation to phase response curves as indication of the structure of the circadian rhythm regulator. A preliminary investigation has shown that the form of this relation may reflect the form of coupling among individual oscillatory units. This will be correlated with the indication about coupling provided by the phenomena listed in (1). Experiments will be designed for further testing of such conclusions. The proposed study will use available data on as many species as possible in order to find the broadest set of phenomena which can be explained by each hypothesis. The main tools of the investigation will be the mathematics of nonlinear systems and computer simulation.